1. Field of the Invention
The present invention relates to a discharge lamp lighting apparatus for controlling lighting of a discharge lamp, particularly a high-pressure discharge lamp for use in a vehicle headlight.
2. Description of the Background Art
There have been proposed various types of discharge lamp lighting apparatuses configured to step up an output voltage of a vehicle-mounted battery into a high voltage by use of a transformer, invert the polarities of the high voltage by use of an inverter in order to light a high-pressure discharge lamp mounted on a vehicle as a headlight on an alternating basis. For example, refer to Japanese Patent Application Laid-Open No. 8-321389.
In such control apparatuses, the electric power supplied to the lamp is adjusted by performing PWM (Pulse Width Modulation) control on a switch device lying on a current path over which the primary current of the transformer flows in accordance with a predetermined control curve defining a relationship between a lamp voltage and a lamp current.
To sum up, an output voltage of a DC-DC converter, which is applied to an H-bridge constituting the inverter, makes the lamp voltage. This lamp voltage is used as the basis of calculation of the electric power supplied to the lamp.
Lamps having a rated power of 35 W, a rated lamp voltage of 85V, and a rated lamp current 0.41A have been conventionally used. To use such a lamp as a vehicle headlight, it is necessary to boot up the light beam, or to make the lamp bright promptly after turning on a lighting switch, and so the lamp is supplied with electric power greater than the rated power in the initial lighting stage.
To give an actual example for a conventional 35 W-lamp (D2S bulb or D2R bulb), the lamp power is controlled such that it is about 75 W in the initial lighting stage, and is decreased gradually to the rated power of 35 W in the stable lighting stage. This lamp power control is performed in accordance with a prescribed control curve defining a relationship between the lamp voltage and the lamp current. For example, by setting the lamp voltage at about 27V in the initial lighting stage and at 85 V in the stable lighting stage, that is, by raising the lamp voltage by 85−27=58 (V), the lamp power can be changed from 75 W to 35 W.
Incidentally, in consideration of environmental pollution, it is desirable to use mercury-less or mercury-free lamps instead of conventional lamps containing a trace quantity of mercury.
In the case of using the mercury-less lamp as a vehicle headlight, it is also necessary to boot up the light beam, or to make the lamp bright promptly after turning on the lighting switch. Accordingly the mercury-less lamp has to be supplied with electric power greater than its rated power in the initial lighting stage. Generally, when using a 35 W-lamp of the mercury-less type, electric power of about 90 W is supplied to the lamp in the initial lighting stage, and is decreased gradually to 35 W in the stable lighting stage. The lamp voltage of the mercury-less lamp in the stable lighting stage is approximately half the voltage lamp of the conventional lamp in the stable lighting stage, whereas the lamp voltage of the mercury-less lamp in the initial lighting stage is approximately equal to the lamp voltage (27V) of the conventional lamp in the initial lighting stage.
When the lamp power control is performed for the mercury-less lamp by use of the above mentioned control curve as in prior art, the lamp power (electric power supplied to the lamp) is set at 90 W in the initial lighting stage and is decreased gradually to be 35 W in the stable lighting stage by changing the lamp voltage from 27V to 42V. In the conventional lamp, the voltage variation required for changing the lamp power by 75 W−35 W=35 W is 85V−27V=58V, whereas in the mercury-less lamp, the voltage variation required for changing the lamp power by 90 W−35 W=55 W is 42V−27V=15V. The ratio of the lamp power variation to the lamp voltage variation in the mercury-less lamp's case is greater than that in the conventional lamp's case.
The above-described lamp voltage variation and the lamp power variation will be explained below in more detail by way of an example.
The lamp voltage used as the basis of calculating the lamp power is a voltage outputted from the DC-DC converter and applied to the H-ridge constituting the inverter as disclosed in the Japanese Patent Application Laid-Open No. 8-321389. To be more precise, a voltage drop across the lamp itself (referred to as “true lamp voltage” hereinafter) added by other voltage drops across other devices such as switch devices, a high-voltage generating coil, etc makes the lamp voltage which is used as the basis of calculation of the lamp power.
The following shows the value of the lamp voltage for each of the case of using the conventional lamp and the case of using the mercury-less lamp, assuming that the inverter (H bridge) is constituted by MOS transistors having on resistance of 0.7 ohms and the coil resistance of the high-voltage generating coil is 1.5 ohms.
In the conventional lamp's case, the lamp current is 2.6A in the initial lighting stage where the electric power supplied to the lamp is 70 W and the true lamp voltage is 27V, while it is 0.41A in the stable lighting state where the electric power supplied to the lamp is 35 W and the true lamp voltage is 85V. The voltage applied to the inverter in the initial lighting stage is calculated according to the following equation 1.27+(0.7×2×2.6)+(1.5×2.6)=34.54(V)  Equation 1
The voltage applied to the inverter in the initial lighting stage is calculated as follows.85+(0.7×2×0.41)+(1.5×0.41)=86.2(V)  Equation 2
Thus, the variation of the voltage applied to the inverter is 86.2−34.54=51.7V.
In the mercury-less lamp's case, the lamp current is 3.3A in the initial lighting stage where the electric power supplied to the lamp is 90 W and the true lamp voltage is 27V, while it is 0.83A in the stable lighting stage where the electric power supplied to the lamp is 35 W and the true lamp voltage is 42V. The voltage applied to the inverter in the initial lighting stage is calculated as follows.27+(0.7×2×9.3)+(1.5×3.3)=36.57(V)  Equation 3
The voltage applied to the inverter in the stable lighting stage is calculated as follows.42+(0.7×2×0.83)+(1.5×0.83)=44.4(V)  Equation 4
Thus, the variation of the voltage applied to the inverter is 44.4−36.57=7.83V
As described above, in the conventional lamp's case, the variation of the voltage applied to the inverter is 51.7V which is smaller by about 6% than the variation of the true lamp voltage which is 58V, since the voltage applied to the inverter includes not only the voltage drop across the lamp, but also the voltage drops across the devices other than the lamp. However, since the variation of the voltage applied to the inverter, which is 51.7V, is relatively large, the contribution ratio of the voltage drops across the devices other than the lamp inn the variation of the lamp voltage are relatively small. Accordingly, it is possible to control the lamp power accurately without difficulty by use of a lamp power calculating circuit designed with consideration given to the effects of the voltage drops across the devices other than the lamp and device-to-device variation.
On the other hand, in the mercury-less lamp's case, the variation of the voltage applied to the inverter is 7.83V which is smaller by about 48% than the variation of the true lamp voltage which is 15V. Since the variation of the voltage applied to the inverter, which is 7.83V, is relatively small, the contribution ratio of the voltage drops across the devices other than the lamp in the variation of the lamp voltage are relatively large. As explained above, to control the electric power supplied to the mercury-less lamp by controlling the voltage applied to the inverter as in prior art, it is necessary to vary the lamp power by 55 W by varying the voltage supplied to the inverter by the value as small as 7.83V. Accordingly, it is difficult to control the lamp power accurately by use of the lamp power calculating circuit even if it is designed with consideration given to the effects of the voltage drops across the devices other than the lamp and device-to-device variation.
As explained above, if the variation of the voltage supplied to the inverter is large as in the conventional lamp's case, the effects of the voltage drops across the devices other than the lamp on the lamp power control are small, since the contribution ratio of the voltage drops across the devices other than the lamp in the lamp voltage variation is small.
However, if the variation of the voltage supplied to the inverter is small as in the mercury-less lamp's case, the effects of the voltage drops across the devices other than the lamp on the lamp power control is large, since the contribution ratio of the voltage drops across the devices other than the lamp in the lamp voltage variation is large.
As a result, there arises a problem in that the build up characteristic of the vehicle headlight's beam defined in the regulation cannot be satisfied by controlling the voltage applied to the inverter as in prior art in the mercury-less lamp's case.